1. Field of the Invention
The present invention relates to drilling fluid telemetry systems and, more particularly, to a telemetry system incorporating an oscillating shear valve for modulating the pressure of a drilling fluid circulating in a drill string within a well bore.
2. Description of the Related Art
Drilling fluid telemetry systems, generally referred to as mud pulse systems, are particularly adapted for telemetry of information from the bottom of a borehole to the surface of the earth during oil well drilling operations. The information telemetered often includes, but is not limited to, parameters of pressure, temperature, direction and deviation of the well bore. Other parameter include logging data such as resistivity of the various layers, sonic density, porosity, induction, self potential and pressure gradients. This information is critical to efficiency in the drilling operation.
Mud pulse valves must operate under extremely high static downhole pressures, high temperatures, high flow rates and various erosive flow types. At these conditions, the valve must be able to create pressure pulses of around 100-300 psi.
Different types of valve systems are used to generate downhole pressure pulses. Valves that open and close a bypass from the inside of the drill string to the wellbore annulus create negative pressure pulses, for example see U.S. Pat. No. 4,953,595. Valves that use a controlled restriction placed in the circulating mud stream are commonly referred to as positive pulse systems, for example see U.S. Pat. No. 3,958,217.
The oil drilling industries need is to effectively increase mud pulse data transmission rates to accomodate the ever increasing amount of measured downhole data. The major disadvantage of available mud pulse valves is the low data transmission rate. Increasing the data rate with available valve types leads to unacceptably large power consumption, unacceptable pulse distortion, or may be physically impractical due to erosion, washing, and abrasive wear. Because of their low activation speed, nearly all existing mud pulse valves are only capable of generating discrete pulses. To effectively use carrier waves to send frequency shift (FSK) or phase shift (PSK) coded signals to the surface, the actuation speed must be increased and fully controlled.
Another example for a negative pulsing valve is illustrated in U.S. Pat. No. 4,351,037. This technology includes a downhole valve for venting a portion of the circulating fluid from the interior of the drill string to the annular space between the pipe string and the borehole wall. Drilling fluids are circulated down the inside of the drill string, out through the drill bit and up the annular space to surface. By momentarily venting a portion of the fluid flow out a lateral port, an instantaneous pressure drop is produced and is detectable at the surface to provide an indication of the downhole venting. A downhole instrument is arranged to generate a signal or mechanical action upon the occurrence of a downhole detected event to produce the above described venting. The downhole valve disclosed is defined in part by a valve seat having an inlet and outlet and a valve stem movable to and away from the inlet end of the valve seat in a linear path with the drill string.
All negative pulsing valves need a certain high differential pressure below the valve to create sufficient pressure drop when the valve is open. Because of this high differential pressure, negative pulse valves are more prone to washing. In general, it is not desirable to bypass flow above the bit into the annulus. Therefore it must be ensured, that the valve is able to completely close the bypass. With each actuation, the valve hits against the valve seat. Because of this impact, negative pulsing valves are more prone to mechanical and abrasive wear than positive pulsing valves.
Positive pulsing valves might, but do not need to, fully close the flow path for operation. Positive poppet type valves are less prone to wear out the valve seat. The main forces acting on positive poppet valves are hydraulic forces, because the valves open or close axially against the flow stream. To reduce the actuation power some poppet valves are hydraulically powered as shown in U.S. Pat. No. 3,958,217. Hereby the main valve is indirectly operated by a pilot valve. The low power consumption pilot valve closes a flow restriction, which activates the main valve to create the pressure drop. The power consumption of this kind of valve is very small. The disadvantage of this valve is the passive operated main valve. With high actuation rates the passive main valve is not able to follow the active operated pilot valve. The pulse signal generated is highly distorted and hardly detectable at the surface.
Rotating disc valves open and close flow channels perpendicular to the flow stream. Hydraulic forces acting against the valve are smaller than for poppet type valves. With increasing actuation speed, dynamic forces of inertia are the main power consuming forces. U.S. Pat. No. 3,764,968 describes a rotating valve for the purpose to transmit frequency shift key (FSK) or phase shift key (PSK) coded signals. The valve uses a rotating disc and a non-rotating stator with a number of corresponding slots. The rotor is continuously driven by an electrical motor. Depending on the motor speed, a certain frequency of pressure pulses are created in the flow as the rotor intermittently interrupts the fluid flow. Motor speed changes are required to change the pressure pulse frequency to allow FSK or PSK type signals. There are several pulses per rotor revolution, corresponding to the number of slots in the rotor and stator. To change the phase or frequency requires the rotor to increase or decrease in speed. This may take a rotor revolution to overcome the rotational inertia and to achieve the new phase or frequency, thereby requiring several pulse cycles to make the transition. Amplitude coding of the signal is inherently not possible with this kind of continuously rotating device. In order to change the frequency or phase, large moments of inertia, associated with the motor, must be overcome, requiring a substantial amount of power. When continuously rotated at a certain speed, a turbine might be used or a gear might be included to reduce power consumption of the system. On the other hand, both options dramatically increase the inertia and power consumption of the system when changing form one to another speed for signal coding.
The aforesaid examples illustrate some of the critical considerations that exist in the application of a fast acting valve for generating a pressure pulse. Other considerations in the use of these systems for borehole operations involve the extreme impact forces, dynamic (vibrational) energies, existing in a moving drill string. The result is excessive wear, fatigue, and failure in operating parts of the system. The particular difficulties encountered in a drill string environment, including the requirement for a long lasting system to prevent premature malfunction and replacement of parts, require a robust and reliable valve system.
The methods and apparatus of the present invention overcome the foregoing disadvantages of the prior art by providing a novel mud pulse telemetry system utilizing a rotational oscillating shear valve.
The present invention contemplates a mud pulse telemetry system utilizing an oscillating shear valve system for generating pressure pulses in the drilling fluid circulating in a drill string in a well bore. One aspect of the invention includes a tool housing adapted to be inserted in the drill string near the bit. Mounted in the tool housing is an oscillating shear valve system comprising a non-rotating stator and a rotationally oscillating rotor, the stator and rotor having a plurality of length wise flow passages for channeling the flow. The rotor is connected to a drive shaft disposed within an lubricant filled pulser housing, and is driven by an electrical motor. A seal prevents wellbore fluid from entering the lubricant filled housing. The motor is powered and controlled by an electronics module. The rotor is powered in a rotationally oscillating motion such that the rotor flow passages are alternately aligned with the stator flow passages and then made to partially block the flow from the stator flow passages thereby generating pressure pulses in the flowing drilling fluid.
In another aspect, the invention includes a flexible elastomeric bellows seal to seal between the rotationally oscillating shaft and the lubricant filled housing.
In one embodiment, the oscillating shear valve is controlled by a processor in the electronics module according to programmed instructions.
In one embodiment, the electronics module senses pressure readings from pressure sensors mounted in the tool housing at locations above and below the pulser assembly. The processor in the electronics module acts to control the differential pressure as indicated by the sensors, according to programmed instructions.
In another embodiment, the electronics module uses the tool housing mounted pressure sensors to receive surface generated pressure command signals, and to modify the downhole encoding based on the surface generated commands.
In another embodiment, a torsional spring is attached to the motor and the end of the pulser housing, the spring being designed such that the combination of the spring and the rotating masses create a torsionally resonant spring-mass system near the desired operating frequency of the pulser. In one aspect of the invention, the torsional spring is a torsion rod type spring. In yet another aspect, the torsional spring is a magnetic spring.
In one embodiment, a method is described for generating a fast transition in a mud pulse telemetry scheme utilizing phase shift key encoding (PSK), comprising, using an oscillating shear valve to generate pressure pulses; driving the oscillating rotor at a first predetermined phase relationship, and changing the drive signal, at a predetermined rotor speed, to a second predetermined phase relationship, and attaining the second predetermined phase relationship in no more than one oscillatory period.
In another embodiment, a method is described for generating a fast transition in a mud pulse telemetry scheme utilizing frequency shift key encoding (FSK), comprising, using an oscillating shear valve to generate pressure pulses; driving the oscillating rotor at a first predetermined frequency, and changing the drive signal, at a predetermined rotor speed, to a second predetermined frequency, and attaining the second predetermined frequency in no more than one oscillatory period.
In one embodiment, a method is described for generating a fast transition in a mud pulse telemetry scheme utilizing amplitude shift key encoding (ASK), comprising, using an oscillating shear valve to generate pressure pulses, driving the oscillating rotor to a first predetermined rotational angle to generate a first signal amplitude, and changing the drive signal, at a predetermined rotor speed, to drive the rotor to a second predetermined rotational angle to generate a higher or lower pulse amplitude than the first signal amplitude.
In another embodiment, a method is described for increasing the data transmission rate of a mud pulse telemetry system by using a combination of FSK and ASK signals to transmit data, comprising, using an oscillating shear valve to generate pressure pulses; driving the oscillating rotor at a first predetermined frequency and first predetermined rotational angle, and changing the drive signal, at a predetermined rotor speed, to simultaneously change to a second predetermined frequency at a second predetermined rotational angle, and attaining the second predetermined frequency and second predetermined rotational angle in no more than one oscillatory period.
In another embodiment, a method is described for increasing the data transmission rate of a mud pulse telemetry system by using a combination of PSK and ASK signals to transmit data, comprising, using an oscillating shear valve to generate pressure pulses; driving the oscillating rotor at a first predetermined phase angle and through a first predetermined rotational angle, and changing the drive signal, at a predetermined rotor speed, to simultaneously change to a second predetermined phase angle at a second predetermined rotational angle, and attaining the second predetermined phase angle and the second predetermined rotational angle in no more than one oscillatory period.
In one embodiment, a method is described for preventing jamming of a mud pulse valve by a foreign body in a fluid stream. The method comprises utilizing an oscillating shear valve to generate pressure pulses, the oscillating shear valve comprising a non-rotating stator and an oscillating rotor, where the rotor is adapted to rotate in a first direction and a second direction, where the second direction is opposite the first direction. The oscillating action of the rotor facilitates washing out any foreign bodies lodged between the rotor and stator.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.